This project, entitled E-Cigarette Use and Longitudinal Changes in Cigarette Smoking and Health proposes to use three waves of nationally representative longitudinal data to examine the relationships among e-cigarette use, traditional cigarette smoking, other tobacco use, other substance use behaviors and health problems from adolescence to young adulthood. E-cigarette use has increased significantly between 2011 and 2014 among U.S. adolescents and has surpassed the current use of traditional cigarette smoking among adolescents. In 2014, more than one in every six U.S. high school seniors used e- cigarettes in the past month. Never-smoking adolescents who have used e-cigarettes are twice more likely to say they intend to smoke traditional cigarettes than never-smoking youth who have not used e-cigarettes. Traditional cigarette smoking is most prevalent among young adults following high school (ages 18-24) relative to all other age groups. Tobacco use remains the leading preventable cause of death, and traditional cigarette smoking is responsible for more than 480,000 deaths per year in the U.S. This raises important public health questions regarding the long-term effects of e-cigarette use: 1) Does e-cigarette use change the risk for subsequent traditional cigarette smoking, other tobacco use, and health problems? 2) Are traditional cigarette smokers more likely to quit smoking after initiating e-cigarette use? To date, the long-term patterns and consequences associated with e-cigarette use in nationally representative samples of adolescents have not been studied. To address these gaps in knowledge, we will conduct a secondary analysis of data from the Monitoring the Future (MTF) study, a multi-cohort epidemiological survey of a sample of approximately 142,500 students enrolled in U.S. private/public secondary schools. The MTF study also features a longitudinal panel sample of high school seniors (modal age: 18 years) who were followed 1-2 years (modal ages: 19-20 years) and 3-4 years (modal ages: 21-22 years) after high school, resulting in three waves of longitudinal data. Several new questions were recently added to the MTF study to assess e-cigarette use. As a result, the MTF has sufficient measures and sample size to test for potential gender, race/ethnicity, and socioeconomic status differences in e-cigarette use and its consequences. Our study aims to: (1) estimate the bias in estimated rates of e-cigarette use and traditional cigarette smoking in adolescence and young adulthood and estimated rates of change over time in e-cigarette use and traditional cigarette smoking due to differential attrition in the MTF study, by comparing and evaluating alternative weighting and imputation approaches; (2) examine the trajectories of e-cigarette use, traditional cigarette smoking, other tobacco use, other substance use behaviors, and health problems from adolescence to young adulthood as a function of e-cigarette use history at age 18; and (3) assess the effects of age of onset and frequency of e-cigarette use on the trajectories of traditional cigarette smoking, other tobacco use, other substance use behaviors, and health problems.